Certain types of video disc systems utilize disc records wherein information is prerecorded by means of geometric variations in tracks or grooves proximate the surface of the disc. The information is reproduced by means of a signal pickup stylus which engages the track or groove and detects the geometric variations representative of the prerecorded signal. In the capacitance type systems the stylus-record interaction operates to form a time varying capacitance as the geometric variations in a particular track are moved past the stylus by the rotation of the disc, which time varying capacitance forms part of a tuned circuit to amplitude modulate a carrier frequency. The amplitude modulation is thereafter detected and converted to video and audio signals suitable for reproduction on standard television receivers. In the pressure sensitive systems, geometric variations in the groove apply a time varying force to the stylus which is mechanically coupled to a pressure sensitive transducer for conversion to electrical signals.
Video disc systems of this type typically employ disc records having track or groove densities of 6,000 to 10,000 per inch. As a result of such high groove densities, it is difficult to reliably translate the stylus radially across the disc in accordance with normal play. Therefore, the signal pickup stylus is mounted in a carriage assembly driven by motive means for translating the stylus radially across the disc generally synchronously with the rotation of the disc. Because the tracks tend to be slightly eccentric the stylus is mounted within the carriage for limited radial movement of the stylus with respect to the carriage. Such relative movement mechanically biases the stylus mounting arm from its home position and undesirably affects stylus tracking. In order to compensate for this condition, the relative stylus position with respect to the carriage assembly is monitored, and the carriage translation is controlled to maintain the stylus mounting arm in a generally unbiased condition and the stylus centered over the track.
The stylus mounting arm or stylus arm is typically a slender, longitudinal member oriented substantially tangent to the information track currently being played, at the point where the pickup stylus interacts with the track. One end of the stylus arm is compliantly mounted within the carriage to permit lateral and vertical pivotal movement. The pickup stylus mounted to the other end of the stylus arm can thereby track disc eccentricities without leaving the information track.
As is readily imagined, the relatively small track dimensions coupled with the relatively large size of the playback apparatus, i.e., the signal pickup stylus assembly and the carriage assembly, requires that the mechanical dynamics of the system be finely tuned. To insure that during playback the pickup stylus does not easily become disengaged from the track, a positive pressure is provided between the pickup stylus and the record disc. This pressure arises from the weight of the pickup stylus-stylus arm assembly and from the spring forces of a resilient flylead arranged in a prescribed deformation for producing such forces substantially normal to the record disc. The pressure should not be so large as to induce significant stylus or disc record wear, but should be large enough to maintain the stylus and disc in continuous contact to preclude undesirable stylus-disc capacitance variations. The flylead performs the additional function of making electrical connection between the signal pickup stylus and the player electronics. See U.S. Pat. No. 4,030,123 issued to B. K. Taylor and entitled "Stylus Adjustment Apparatus for a Video Disc Player", for an example of a stylus/stylus arm mounting arrangement.
In order to incorporate special effects options in the player, it is desirable to provide a mechanism for selectively steering the stylus to particular tracks very rapidly, e.g., to radially deflect the stylus a particular number of information tracks during the vertical blanking interval of the video signal. A stylus deflection of one track outwardly per disc rotation will provide quasi-stop motion and a one track deflection inwardly per disc rotation will produce a display which appears to be twice normal speed, etc. Prior art players have included magnetic or piezoelectric transducers cooperating with the stylus arm to provide such stylus deflection. U.S. Pat. Nos. 3,963,861; 3,993,863; and 4,164,756 show video disc players having stylus deflector transducers comprising bimorphs which are integral with the respective stylus arms. These devices, however, fail to include means for insuring a prescribed minimum disc-stylus pressure. To this end a leaf spring is incorporated between the stylus arm and the carriage. One consequence of incorporating a leaf spring however is that disc warpage will tend to create vertical translations in the stylus arm varying the elastic deformation of the spring and thereby the stylus-disc pressure created by the spring. It is evident that a stylus engaging a groove becomes more difficult to dislodge as the pressure exerted by the leafspring increases. Considering that the deflector energization pulses will be of nominal or constant amplitude, the amount of lateral deflection will be related to the stylus-disc pressure and can be somewhat unpredictable for discs having significant warpage causing variations in disc-stylus pressure.
Typical of magnetic stylus deflection transducers are U.S. Pat. No. 4,183,059, Copending Applications Ser. No. 039,358 filed June 15, 1979 and Ser. No. 039,359 filed June 15, 1979 (now U.S. Pat. Nos. 4,258,233 and 4,262,174 respectively) and U.S. Pat. No. 3,973,080. Each of these references show respective stylus arms having a permanent magnet secured thereto with electromagnetic coils secured to the carriage apparatus and proximate the permanent magnet so that selective energization of the coil will create a lateral or radially directed magnetic flux to act on the permanent magnet and deflect the stylus arm. The performance of the magnetically coupled transducers tend to be less predictable than direct coupled piezoelectrics whether or not a damper or leafspring is employed. However, when a leafspring or damper is utilized with the magnetic transducer it decreases the predictability of performance when a warped record is played.